Paraelectrical tuning of a charge carrier density as high as 10¹³ cm⁻² in the presence of a high electronic carrier mobility on the delicate surfaces of correlated oxides, is a key to the technological breakthrough of a field effect transistor (FET) utilising the metal-nonmetal transition. Here we introduce the Parylene-C/Ta₂O₅ hybrid gate insulator and fabricate FET devices on single-crystalline SrTiO₃, which has been regarded as a bedrock material for oxide electronics. The gate insulator accumulates up to ~10¹³cm⁻² carriers, while the field-effect mobility is kept at 10 cm²/Vs even at room temperature. Further to the exceptional performance of our devices, the enhanced compatibility of high carrier density and high mobility revealed the mechanism for the long standing puzzle of the distribution of electrostatically doped carriers on the surface of SrTiO₃. Namely, the formation and continuous evolution of field domains and current filaments.